The free energy of classical two-dimensional solids with repulsive inverse-power interactions is obtained from a recently introduced perturbation theory. We obtain very accurate results for the thermodynamics of the solid phase as compared to the available simulation data.

High-quality sizable single crystals of the radical compound 2,4,6-triphenylverdazyl (TPV) have been prepared and their magnetic properties investigated by neutron-diffraction and magnetization measurements. Neutron-diffraction peaks corresponding to long-range antiferromagnetic ordering have been observed below the Néel temperature of 1.78 K. The determined spin structure has intrachain antiferromagnetic coupling extending toward the c axis with the spins aligned to the b axis. Magnetization measurements also reveal that weak ferromagnetism appears below ${\mathit{T}}_{\mathit{N}}$ with the ferromagnetic components in directions parallel and perpendicular to the spin direction. The magnetization curve shows no hysteresis, which is consistent with small anisotropy of 2p electrons in organic magnets.

In this work the mean-field renormalization-group approach for a random-bond Ising model with competitive interactions was used in order to obtain the magnetic phase diagram and to compare with previous experimental results for the system (${\mathrm{Fe}}_{0.65}$${\mathrm{Ni}}_{0.35}$${)}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$ [J. Hesse, Hyperfine Interact. 47, 357 (1989)]. We also obtained the paramagnetic, ferromagnetic, antiferromagnetic, spin glass, and reentrant-spin-glass phases. The theoretical magnetic phase diagram is in good agreement with the experimental results.

The final-state rule within the local-density approximation is used to explore the effects on the x-ray absorption spectra of a static-spin-polarized core hole in the 2${\mathit{p}}_{3/2}$ states of 3d-transition-metal ferromagnets. The excited atom with a core hole is treated as a single impurity in a lattice within the supercell geometry approach. We have found that the photoelectron localizes on the impurity site mostly as a minority spin substantially reducing the local spin magnetic moment. The position and the shape of the ${\mathit{L}}_3$ ‘‘white line’’ of absorption spectrum changes shape with respect to that calculated for the initial state rule, i.e., without the core hole.

We present a Landau-type semiphenomenological theory for the zero-sound dispersion relation in normal-liquid${\mathrm{-}}^3$He films in the presence of a magnetic field or for partially polarized films. We express our result in terms of the spin-dependent (${\mathit{F}}_0^{\mathit{a}}$) and spin-independent (${\mathit{F}}_0^{\mathit{s}}$) first Landau parameters. We give the zero-field result and the lowest-field correction. We find that the field effect should be readily observable for the presumably attainable experimental accuracy.

Behavior of the isotope effect when the superconducting ${\mathit{T}}_{\mathit{c}}$ becomes large on account of an attractive, short-range interaction is reviewed. We explain the difference between results obtained using the two-square-well model and those found by using McMillan’s equation.

We solve exactly a ${\mathrm{Cu}}_4$${\mathrm{O}}_8$ cluster with four and five holes, for parameters for which the low-energy physics of the three-band Hubbard model can be mapped exactly to a t-J model with J=0. When a Cu-Cu superexchange J is included, the mapping is no more exact. We investigate the quality of the mapping and the amount of nonorthogonal Zhang-Rice singlets of each low-energy eigenstate as a function of J. We also explain briefly how to calculate a property of the three-band model (such as the Cu and O photoemission intensity) using the t-J model.

A Frank-Kaspar phase-related periodic model of V-Ni is set up. It explains well the experimental data of the dodecagonal phase [see Phys. Rev. Lett. 60, 1645 (1988)] but not the corresponding unclear high-resolution electron-microscopic image. The dodecagonal phase can be understood to be that of a perturbed nonclassical crystal.

Magnetoresistance oscillations with a period of two monolayers of Cr have been observed in Fe/Cr/Fe trilayers. Magneto-optic measurements were performed on the same sample in order to verify the periodicity and determine the nature of the coupling between the Fe layers. The saturation fields obtained from both the transport and optical measurements agree well at room temperature. These results indicate that the magnetoresistance is controlled by the coupling mechanism.

Measurements of the short-range-order (SRO) diffuse scattered intensity show peaks at the 〈11/20〉 and 〈100〉 points in ${\mathrm{Ni}}_3$V and ${\mathrm{Pd}}_3$V, respectively, although the stable ground state in both systems (D$0_{22}$) is a 〈11/20〉-type structure. Mean-field theory predicts 〈11/20〉 SRO in both materials, in contradiction with experiment for ${\mathrm{Pd}}_3$V. The 〈100〉-type SRO in ${\mathrm{Pd}}_3$V has been explained previously as a non-mean-field effect. Via a combination of first-principles total-energy calculations and Monte Carlo simulated annealing, we show that non-mean-field effects are insufficient to explain the observed SRO of ${\mathrm{Pd}}_3$V. However, the inclusion of electronic excitations leads to a temperature dependence in the interaction energies which correctly explains both the SRO and phase stability in ${\mathrm{Pd}}_3$V and ${\mathrm{Ni}}_3$V.

Existing theories of the oscillatory exchange coupling in magnetic metallic multilayers are reconciled within a common approach. It is shown that Ruderman-Kittel-Kasuya-Yosida–type theories and full-confinement quantum-well results are obtained as limiting cases. Numerically useful and model-independent closed-form expressions for the exchange coupling across a nonmagnetic spacer are obtained in terms of the one-electron propagators. The theory can be extended to deal with nonmagnetic alloy spacers and it is argued that the existence of disorder in the spacer may lead to significant reduction of the interlayer coupling.

We report a calculation of the quasiparticle interactions in a 1/N expansion of the infinite-U Anderson lattice. With an anisotropic hybridization matrix element, V(k→), as sites of cubic symmetry and crystal-field splitting of J=5/2 spin-orbit-coupled multiplets, we average the static quasiparticle scattering amplitude over the Fermi surface. We find the local interactions are repulsive and have an appreciable strength only in the ${\mathit{A}}_{1\mathit{g}}$ (s-wave) pairing channel. The absence of strong local repulsions in the remaining pairing channels of cubic symmetry: ${\mathit{E}}_{\mathit{g}}$(${\mathit{x}}^2$-${\mathit{y}}^2$), ${\mathit{T}}_{1\mathit{g}}$[xy(${\mathit{x}}^2$-${\mathit{y}}^2$)], and ${\mathit{T}}_{2\mathit{g}}$(xy) increases the prospects of a superconducting pairing instability in these channels. Inclusion of the nonlocal corrections suggests the possibility of superconductivity of ${\mathit{T}}_{1\mathit{g}}$ symmetry for a wide range of parameters, and ${\mathit{E}}_{\mathit{g}}$ symmetry superconductivity for some model parameter regions.

The low-frequency shot-noise power of a normal-metal–superconductor junction is studied for an arbitrary normal region. Through a scattering approach, a formula is derived that expresses the shot-noise power in terms of the transmission eigenvalues of the normal region. The noise power divided by the current is enhanced by a factor 2 with respect to its normal-state value, due to Cooper-pair transport in the superconductor. For a disordered normal region, it is still smaller than the Poisson noise, as a consequence of noiseless open scattering channels.

We have calculated the temperature-dependent shear modulus μ(T) of a vortex lattice in a two-dimensional superconductor. We include fluctuations by expanding the Ginzburg-Landau free-energy functional in states from the lowest Landau level, and evaluate the shear modulus by standard Monte Carlo techniques for rectangular samples containing up to 224 vortices. At the thermodynamic melting transition, which is apparently first order as found by previous workers, the shear modulus falls from a finite value in the solid phase to zero in the liquid. Despite the first-order transition, the magnitude of the drop may be near the universal value envisioned in dislocation-unbinding theories.

Exact-diagonalization studies for a doped t-J ladder (or double chain) show hole pairing in the ground state. The excitation spectrum separates into a limited number of quasiparticles which carry charge +‖e‖ and spin 1/2 and a triplet mode. At half-filling the former vanish but the latter evolves continuously into the triplet band of the spin liquid. At low doping the quasiparticles form a dilute Fermi gas with a strong attraction but simultaneously the Fermi wave vector, as would be measured in photoemission, is large.

The low-temperature specific heat, C(T), of the Delafossite family of triangle-planar cuprates, R${\mathrm{CuO}}_{2+\mathrm{\delta }}$, has been studied for R=La and Y and 0<δ≤2/3. Structural transformations between the hexagonal and the orthorhombic phases for R=Y appear as distinct lattice-contribution changes in C(T). Near the oxygen-ordered limit, δ=2/3, C(T) displays a linear-in-T term of 6.5 and 1.3 mJ/mole ${\mathrm{K}}^2$ for R=La and Y, respectively. The large specific-heat enhancement and a similar enhancement of the magnetic susceptibility for R=La are interpreted in terms of strong correlations near a metal–antiferromagnetic-insulator instability.

The Cini-Sawatzky theory of CVV Auger transitions does not give simultaneous agreement with the position and line shape of the ${\mathit{M}}_{45}$${\mathit{N}}_{45}$${\mathit{N}}_{45}$ Auger spectrum of Ag. In accord with recent results for Au, this failure is attributed to the neglect of off-site interactions between the final-state holes. The magnitude of the discrepancy between theory and experiment for Ag is twice that found for Au and this is understood in terms of the difference in the ratio of off-site interaction to d-band width in the two metals.

The transport and magnetic properties of a series of ${\mathrm{Nd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$ perovskites where 0≤x≤0.8 were investigated. The transition from a Mott-Hubbard insulator (${\mathrm{NdTiO}}_3$) with a 3${\mathit{d}}^1$ configuration to a metallic state was observed to take place near x=0.15. Low-field susceptibility and remanent-moment measurements suggest that ${\mathrm{Nd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$ orders antiferromagnetically in the composition region 0≤x≲0.15. The metallic ${\mathrm{Nd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$ compounds behave as Fermi-liquid systems where the effective mass increases as the metal-insulator boundary (x≊0.15) is approached.

We present high-resolution measurements of directional Compton profiles in hexagonal BN (hBN), using inelastic x-ray scattering. The results are compared with the corresponding profiles obtained from pseudopotential energy-band calculations. BN is an important case for comparison with similar studies in metals and other nearly two-dimensional semimetallic solids. Despite the highly anisotropic nature of hBN, we find that the profile anisotropy is not large. There is an excellent agreement between theoretical and experimental profile anisotropies, substantially better than the case of graphite.

The generalization of the variation of the action-integral operator introduced by Schwinger in the derivation of the principle of stationary action enables one to use this principle to obtain a description of the quantum mechanics of an open system. It is shown that augmenting the Lagrange-function operator by the divergence of the gradient of the density operator, a process which leaves the equations of motion unaltered, leads to a class of generators whose associated infinitesimal transformations yield variations of the action-integral operator for an open system, similar in form and content to those obtained for the total, isolated system. The augmented Lagrange-function operator and the associated action-integral operator are termed proper operators, since only their variation yields equations of motion for the observables of an open system, in agreement with the expressions obtained from the field equations. Modifying the generator in this manner is shown to be equivalent to requiring that the open system be bounded by a surface through which there is a local zero flux in the gradient vector field of the electron density. Only the observables of such properly defined open systems are described by the correct equations of motion. The physical significance of such proper open systems is discussed.